A common practice to increase production from a crude oil or gas well involves an acid stimulation treatment of the well. Acid stimulation of a well involves the pumping downhole of an aqueous acid solution which reacts with the subterranean hydrocarbon containing formations, such formations usually consisting of limestone or sand, to increase the size of the pores within the formations and provide enlarged passageways for the crude hydrocarbons to more freely move to collection points which otherwise would be obstructed. Unfortunately, during such acidizing operations asphaltene containing asphaltic sludge precipitates, blocking the existing and newly formed passageways and reducing the efficacy of the acidizing treatment. The formed asphaltic sludges vary in appearance depending on the types of additives used, amount of iron present, and the nature of the crude. In addition, crude hydrocarbons contain chemicals which stabilize emulsions formed upon contact with the aqueous acid during an acid stimulation treatment. Such emulsions are undesirable because they increase the viscosity of the pumped fluid and impede the flow into and out from the well bore.
Various additives may be included in the acidizing solution to minimize such sludging and emulsification of the hydrocarbon/acid solution. While known anti-sludging compositions are effective for some types of crude hydrocarbons and pumping conditions, they do not consistently perform well with the wide variety of such crudes and conditions encountered in the field, and are especially ineffective where the acidizing fluids include significant concentrations of iron containing ferrous (Fe.sup.+2) and/or ferric (Fe.sup.+3) salts. The most troublesome iron concentrations are the especially high iron concentrations which include iron concentrations of over about 2,000 parts per million (ppm), based on the weight of acidizing fluid, and may reach 35,000 parts per million and even higher.
More specifically, during the acidizing process of stimulating a subterranean hydrocarbon containing formation, contaminants, treating fluids and formation hydrocarbon fluids can interact to form undesirable precipitate sludges. It is well known that strong acids such as hydrochloric, hydrofluoric or hydrochloric/hydrofluoric blends cause sludging upon contact with certain crude oils, particularly crude oils from wells in California, Alaska, Canada, Texas, Oklahoma and Louisiana, among others. The use of strong acids pose more of a sludging problem than weak acids, such as acetic or formic acids; yet, the strong acids are preferred for their superior ability to react with and enlarge passageways in the subterranean formations.
It has been observed that the amounts and types of asphaltenes present in the crude hydrocarbons play a major role in sludge formation. Asphaltenes may be present in crude hydrocarbons in the form of a colloidal dispersion which consists of an aggregate of polyaromatic molecules surrounded and stabilized by lower molecular weight neutral resins and paraffinic hydrocarbons. Strong acids from the stimulating treatment seem to destabilize the asphaltic colloidal dispersion and thus cause asphaltenic sludge precipitates and rigid film emulsions. This strong acid/crude oil interaction can be readily observed by placing a drop of oil on top of or surrounding a drop of acid on a glass slide. A rigid film becomes readily apparent in only a few minutes. Under a microscope, small droplets of acid develop very black irregularly shaped rings around them indicative of the sludging and emulsion-forming processes.
Sludging tendencies of different hydrocarbon crudes vary with structural variations in the stabilizing resin layer or are even based on the length of time a well or field has been producing. Additives which tend to remove the colloidal dispersion stabilizing resins seem to affect the quantity of asphaltene sludge produced. Also, many nonemulsifiers or demulsifiers, as well as surface tension reducing agents, have oil soluble detergent components which serve to destabilize the asphaltene containing colloidal dispersion and increase acid related asphaltenic sludge formation.
The amount of iron contamination in the acidizing fluids is also important in the formation of undesirable sludge. Hydrocarbon crudes, especially those with high asphaltene colloidal stabilizing resin to asphaltene ratios, are known to form insoluble sludge in the presence of acid solutions. In addition, acid solutions with iron concentrations such as 2000 ppm and above are known to form insuluble sludges considerably worse. The iron originates from several sources, including contamination in the technical acid solution itself, leaching from the iron-containing piping and surface pumps used in the process of pumping downhole the acid-containing fluid, as well as from the subterranean rock and mineral formations. The amount of iron present in the acidizing fluid in the well can easily exceed 5,000 ppm and even reach 35,000 ppm and higher. The amount of iron is present in many wells as both ferrous (Fe.sup.+2) and ferric (Fe.sup.+3) ions, in approximately a 3:1 ratio. Ferric (Fe.sup.+3) ions increase the sludging tendencies to a considerably greater extent than ferrous (Fe.sup.+2) ions.
Many chemical compounds and compositions exist which are classified as anti-sludgers, nonemulsifiers or demulsifiers, mutual solvents, iron controllers or corrosion inhibitors and are said to affect sludge formation during well acidizing. For example, Hall U.S. Pat. No. 4,073,344 discloses a method of treating subterranean formations with a composition for reducing the adsorption of treatment additives on the formations. The composition includes ethoxy ethanols in addition to corrosion inhibitors, such as propargyl alcohol, anti-sludge agents such as dodecyl benzenesulfonate (DDBSA), acids, demulsifiers and surfactants. Esters of dodecyl benzenesulfonic acid are disclosed in Looney et al U.S. Pat. No. 4,442,014 as anti-sludging agents which require surfactants such as alkoxylated polyfunctional alcohols, alkoxylated polyols, or alkoxylated alkylphenol-formaldehyde resins. An ethoxylated alkylphenol dissolved in ethylene glycol and methanol is described as a dispersant for an anti-sludging sulfonic acid in Ford U.S. Pat. No. 4,823,874.
Conventional acid treatment systems to enhance oil production have relied mainly on cationic corrosion inhibitors consisting of quaternized amines, and combinations including such quaternized amines along with oxyalkylated alkylphenols, complex polyolesters, formaldehyde and acetylenic alcohols in aromatic hydrocarbons, alcohols or water. Briggs U.S. Pat. No. 4,698,168 discloses a corrosion inhibiting composition containing an acetylenic alcohol, propargyl alcohol, and 2-ethoxy ethanol with minor amounts of a polyethylene glycol, an alkylphenol-formaldehyde resin, quinoliniums and tar bases.
Conventional nonemulsifiers include cationic organic amines or quaternized amines, as well as ethoxylated alkylphenols. Many acidizing systems which incorporate polymeric nonionic additives and cationic additives still tend to form emulsions due to the absorption of the additives onto fine mineral particles which are released from the earth formations during the acidizing treatments.
While such currently used techniques and acidizing mixtures have achieved varying degrees of success, especially where the asphaltene content of the crude is below about 0.5% by weight and the iron content is below about 2000 ppm, the current treatments have failed to produce consistently the necessary anti-sludging properties, especially at higher levels of iron. It is, therefore, a purpose of the present invention to provide new and useful compositions for including in a method of acid stimulation of hydrocarbon wells. Such compositions inhibit or prevent the formation of asphaltene sludges in the presence of treating acids with relatively high concentrations of ferric and ferrous ions.